Whipping

Whipping is the transient hull-girder vibration that follows an impulsive load such as bottom slamming, bow-flare slamming, or green-water impact, in which the ship rings at its lowest vertical bending natural frequency (typically the two-node mode) and decays over several cycles. The vibratory bending moment superimposes on the wave-frequency moment and can raise the peak sagging or hogging moment by 15 to 40 percent, so it is added to the rule wave load for extreme-strength and fatigue checks. It is distinguished from springing, which is a steady resonant response to wave excitation, whereas whipping is impulse-driven and transient. Monitoring uses strain gauges and the random decrement or modal-fit techniques.